1. Field of the Invention
The present invention relates to a pressure member suitable for use in a heat fixing apparatus to be mounted on an image forming apparatus selected from the group consisting of an electrophotographic copier and an electrophotographic printer and relates to an image heating apparatus including the pressure member.
2. Description of the Related Art
A heat fixing apparatus to be mounted on a printer of an electrophotographic system and a photocopier of a heat roller system includes a halogen heater, a fixing roller heated by the halogen heater, and a pressure roller brought into contact to the fixing roller to form a nip portion. In addition, a heat fixing apparatus of a film heating system includes a heater including a heat generating resistance body on a substrate made of ceramics, a fixing film contacting the heater to move, and a pressure roller forming a nip portion with the heater through the fixing film.
When a printer on which a fixing apparatus of the above described heat roller system is mounted prints a small sized recording material continuously at the same print interval as the interval in the case of large sized recording material, a phenomenon in which the temperature rises too much in a region (non-sheet feeding region) where recording material does not pass (temperature rises in the non-sheet feeding region) in a longitudinal direction of a fixing nip portion occurs. When the temperature rises too much in the non-sheet feeding region, respective parts configuring the fixing apparatus can be damaged. In addition, printing a large sized recording material in the state where the temperature rises too much in the non-sheet feeding region, causes a region of the recording material corresponding to the non-sheet feeding region to be heated more than necessary. Therefore, high-temperature offset will take place.
In particular, in the case of a film heating type heater capable of using a low heat capacity ceramic heater as a heating body, the heat capacity of the heating body is smaller than the heat capacity of the heat roller system. Therefore, the temperature significantly rises in the non-sheet feeding portion of the heating body and the endurance of the pressure roller deteriorates and high temperature offset is likely to occur. In addition, problems such as film drive instability and film wrinkling are likely to occur.
In addition, as the process speed of the printer gets faster, the temperature is likely to rise too much in the non-sheet feeding region. The reason is that an intensive increase in speed is accompanied by a shortening in the time for the recording material to pass the nip portion and, therefore, the fixing temperature required for heat fixing a toner image onto the recording material cannot be prevented from being made higher. In addition, the phenomenon that the time when no recording material is present in the nip portion during a continuous print step (so-called sheet absent time) decreases, accompanies an intensive increase in speed of the printer and, therefore, the unevenness of temperature distribution is hardly averaged during the time when the recording material is present between sheets.
As a unit for reducing the temperature rise in the non-sheet feeding portion, a technique of enhancing thermal conductivity of a pressure roller is generally known. An advantage of this approach is that an improvement in heat transfer in an elastic layer which the pressure roller includes can give rise to a decrease in the temperature rise in the non-sheet feeding portion, that is, the difference in heat in the longitudinal direction of the pressure roller decreases.
Japanese Patent Application Laid-Open No. H11-116806, Japanese Patent Application Laid-Open No. H11-158377 and Japanese Patent Application Laid-Open No. 2003-208052 disclose that highly thermal conductive filler selected from the group consisting of alumina, zinc oxide and silicon carbide is added to base rubber in order to improve thermal conductivity of the elastic layer of the fixing roller and pressure roller.
Japanese Patent Application Laid-Open No. 2002-268423 discloses a method of causing an elastic layer to contain carbon fiber in order to improve thermal conductivity of a rotator (not a pressure roller but a fixing belt, though) including an elastic layer.
Japanese Patent Application Laid-Open No. 2000-39789 discloses an invention of causing an elastomer layer to contain anisotropic filler such as graphite for improving thermal conductivity in the roller thickness direction.
Japanese Patent Application Laid-Open No. 2002-351243 discloses an invention of providing a layer of fabric with pitch based carbon fiber in an elastic layer of a pressure roller.
Japanese Patent Application Laid-Open No. 2005-273771 discloses an invention of dispersing pitch based carbon fiber across a pressure roller elastic layer.
However, even if filler selected from the group consisting of alumina, zinc oxide, silicon carbide, carbon fiber and graphite as described in a publication selected from the group consisting of Japanese Patent Application Laid-Open No. H11-116806, Japanese Patent Application Laid-Open No. H11-158377, Japanese Patent Application Laid-Open No. 2003-208052, Japanese Patent Application Laid-Open No. 2002-268423 and Japanese Patent Application Laid-Open No. 2000-39789, is added to an elastic layer for an increase in thermal conductivity, the desired thermal conductivity cannot be obtained in the case of the addition of a small amount of filler. In addition, in the case of the addition of a large amount of filler, the hardness of the pressure roller gets too large to obtain a desired nip width required for heat fixing a toner image onto recording material, giving rise to a problem. As described above, intensification of thermal conductivity and intensification of hardness of the pressure roller have been hardly established together.
A pressure roller disclosed in Japanese Patent Application Laid-Open No. 2002-351243 is extremely excellent in thermal conductivity. However, due to one of the fabric and the fabric based configuration thereof, the hardness of a highly thermal conductive rubber compound layer will increase. In that case, in order to decrease the hardness of the entire pressure roller, foamed sponge rubber is suitably used for an elastic layer in a lower layer. Accordingly, since the elastic layer in the lower layer is configured by a foamed sponge, there is a room for an improvement in endurance thereof during consumption.
In addition, the pressure roller disclosed in Japanese Patent Application Laid-Open No. 2005-273771 is excellent in thermal conductivity in the longitudinal direction of the roller and a suitable hardness of the roller can be attained, but the heat transfer from the elastic layer to core metal is so high that the roller surface temperature gets too low. In the case where the pressure roller surface temperature is too low, steam appearing where recording material passes a heating nip forms dew on the pressure roller surface to cause instability in the conveyance of the recording material.